Beamforming in a mu-mimo wireless communication system

ABSTRACT

This invention provides methods for Distributed Massive MIMO (DM-MIMO) that use one or more central Baseband Units (BBUs), one or more Multi-User Beamformers for each BBU performing multi-user MIMO computations, and a number of RRHs distributed over a geographic area.

This application claims the benefit of U.S. Provisional Application No.62/104,088 filed on Jan. 16, 2015.

FIELD OF INVENTION

This invention relates to Multi-User Multiple-Input Multiple-Output(MU-MIMO) wireless communications, and more particularly, to beamformingin a MU-MIMO system.

BACKGROUND

To meet the continued fast growing demand of mobile data, the wirelessindustry needs solutions that can provide very high data rates in acoverage area to multiple users simultaneously including at cell edgesat reasonable cost. Currently, the wireless telecom industry is focusedon dense deployment of small cells, the so called ultra-dense networks,to increase spatial re-use of wireless spectrum as the solution formeeting the growing mobile data demand. Dense deployment of small cellsrequires a large number of backhauls and creates highly complexinter-cell interference. One solution to the interference problem is torequire careful Radio Frequency (RF) measurement and planning andinter-cell coordination, which significantly increases the cost ofdeployment and reduces the spectral efficiency. Another solution is theSelf-Organizing Network (SON) technology, which senses the RFenvironments, configures the small cells accordingly throughinterference and Tx management, coordinated transmission and handover.SON reduces the need for careful RF measurement and planning at the costof increased management overhead and reduced spectral efficiency. Thebackhaul network to support a large number of small cells is expensiveto be laid out.

Another method for increasing spatial re-use of wireless spectrum isMIMO, especially Multi-User MIMO (MU-MIMO). In a wireless communicationsystem, a wireless node with multiple antennas, a Base Station (BS) or aUser Equipment (UE), can use beamforming in downlink (DL) or uplink (UL)to increase the Signal-to-Noise Ratio (SNR) orSignal-to-Interference-plus-Noise Ratio (SINR), hence the data rate, ofthe links with other wireless nodes. MU-MIMO can beamform to multipleUEs simultaneously in a frequency and time block, e.g., a Resource Block(RB), i.e., using spatial multiplexing to provide capacity growthwithout the need of increasing the bandwidth. In a large-scale MIMO ormassive MIMO system, a BS may be equipped with many tens to hundreds ofantennas. In order for the BS to beamform to multiple UEs using theplural of antennas, the BS needs to know the DL channels to the UEssufficiently accurately, e.g., the DL Channel State Information (CSI) ofeach UE. However, it is not efficient to obtain the DL CSI directly bysending reference pilots in the downlink because of two reasons: (1).The large number of antennas on the BS would cause large system overheadfor reference signals in the downlink; (2). Dozens of bits are needed toquantize the CSI accurately, which causes overload of the feedbackchannel in the UL. Fortunately, the reciprocal property of an over theair wireless channel, such as in a Time-Division Duplexing (TDD) systemor in a Frequency-Division Duplexing (FDD) system using switching tocreate channel reciprocity as described in our PCT applicationPCT/US14/71752 filed on Dec. 20, 2014, claiming the benefit ofprovisional patent application 61/919,032 filed on Dec. 20, 2013, can beemployed to reduce the channel estimation overhead. In such a system, aUE sends a pilot signal, e.g., Sounding Reference Signal (SRS), which isreceived by all the antennas on the BS in the UL. The BS estimates theUL CSI through the received pilot signal and uses it to estimate the DLCSI based on channel reciprocity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the structure of a Distributed Massive MIMO (DM-MIMO)system.

DETAILED DESCRIPTION

Reference may now be made to the drawings wherein like numerals refer tolike parts throughout. Exemplary embodiments of the invention may now bedescribed. The exemplary embodiments are provided to illustrate aspectsof the invention and should not be construed as limiting the scope ofthe invention. When the exemplary embodiments are described withreference to block diagrams or flowcharts, each block may represent amethod step or an apparatus element for performing the method step.Depending upon the implementation, the corresponding apparatus elementmay be configured in hardware, software, firmware or combinationsthereof. Here after, a pilot signal may mean a signal transmitted by oneantenna for the purpose of estimating the channel between thetransmitting antenna and one or more receiving antennas. It may also becalled a reference signal, a channel estimation signal or a test signal.

In the following descriptions, an antenna is used to indicate a RF paththat includes the RF circuits and the antenna unless indicated by thecontext otherwise, for example, in a hybrid beamforming system, one RFpath may be connected to multiple antenna elements via a beamformingcircuit, mostly analog. In such a system, all the antenna elementsconnected to the same RF path can be treated as a single equivalentantenna in baseband processing.

One embodiment of this invention is a method for wireless networkdensification to provide higher throughput and higher spectrumefficiency than deploying dense small cells. The embodiment is referredto as Distributed Massive MIMO (DM-MIMO), as illustrated in FIG. 1,comprising of a central baseband unit (BBU) supported by a multi-userbeamformer (MU-BFer) as well as a centralized backhaul, and a largenumber (many tens to hundreds) of Remote Radio Heads (RRHs), each ofwhich contains antennas, RF transceivers and sync circuits. They can beplaced in places where small cells are placed or planned, or in otherlocations in the coverage area. Each RRH can have multiple antennas andall RRHs can transmit and receive in the same frequency bands for true“Frequency Reuse 1” in the entire area covered by all the RRHs. A RRHcan be added to wherever improved coverage is needed, without requiringRF planning. Each RRH is connected to the central BBU via a fronthaulconnection, which can be via an optical fiber, electrical cable or awireless link. Either digital IQ samples or analog RF signals arecarried over the fronthaul connections. In the case of sending digitalIQ samples over a fronthaul connection, the central BBU sends a masterreference clock signal via the fronthaul connection to all RRHs, each ofwhich recovers the master reference clock signal and uses it to generatethe local clock and carrier signal to ensure all RRHs use the samecarrier frequency, referred to as frequency synchronization. Inaddition, the central BBU may also calibrate the time delay with eachRRH and adjust it accordingly to ensure signals transmitted by all RRHsare synchronized in time. In one embodiment, the accuracy of timesynchronization is relaxed as long as the time synchronization error iswithin the systems cyclic prefix, and the difference in delays among theRRHs are captured in channel model and taken care of in digital basebandprocessing. This reduces the cost and complexity of time synchronizationon multiple RRHs. In another embodiment, synchronization of carrierphases among the multiple RRHs is not required. Instead, the carrierphase of each RRH is locked to its recovered master reference clock,e.g., using a Phase Locked Loop (PLL) circuit, thus the phasedifferences among the RRHs are fixed and are included in the channelmodel. As a result, these phase differences are handled in digitalbaseband.

Multi-user beamforming (MU-BF) is performed by the MU-BFer, e.g., usingZero-Forcing (ZF), Regularized-ZF (RZF), Minimum Mean Square Error(MMSE), Dirty-Paper Coding (DPC) or Conjugate Beamforing (CB), forantennas on all RRHs or clusters of RRHs to achieve a high order ofspatial multiplexing over the entire coverage area with low inter-beaminterference. The same frequency resource or the whole spectrumallocated to the BS can be simultaneously beamed to many UEs. Whenantennas in a cluster of RRHs (which may contain one or more RRHs, aslong as a sufficient number of antennas are contained in the cluster)are used to perform MU-BF, it is referred to as Distributed MU-BF(DMU-BF). In one embodiment of DMU-BF, channel estimation or MU-BF usingthe same frequency resources are performed simultaneously for clustersof RRHs that are sufficiently far apart (relative to the transmittingpower and large-scale fading), without worrying about interferencesamong the UE pilot signals or beams of different clusters because oftheir spatial separation by deployment. This embodiment is highlyscalable, meaning that a large number of RRHs can be added to theDM-MIMO system to deploy over a coverage area so that the DM-MIMO systemcan beamform the same frequency resource or the whole spectrum allocatedto the BS simultaneously to a very large number of UEs, e.g., tens tohundreds of UEs.

Multiple BBUs and their associated DM-MIMO systems can be deployedadjacently to cover a wider area. A DM-MIMO system can use additionalantennas in overlapping coverage areas to make the transmissions by itsRRHs to be orthogonal to the channels to the UEs in a neighboringDM-MIMO system to reduce the interference to the UE in the neighboringDM-MIMO system. A DM-MIMO system can obtain estimations of channels toUEs in a neighboring DM-MIMO system by listening to the pilot orreference signals transmitted by the UEs, and use the channel estimatesto compute a pre-coding matrix to make the transmissions by its RRHs tobe orthogonal to the channels to the UEs in a neighboring DM-MIMOsystem. This embodiment requires the multiple BBUs and their associatedDM-MIMO systems to be synchronized.

DM-MIMO requires high speed fronthauls if digital IQ samples aretransmitted via the fronthauls. The data rate on such fronthaulstypically will be significantly higher than the data rate of backhaulsfor small cells. Alternatively, the fronthauls can transmit analogsignals using RF-over-Fiber (RFoF). The digital fronthauls between theRRHs and the BBU can use star or cascade connections, or a combinationas illustrated in FIG. 1, which comprises of a central baseband unit(BBU) 1 supported by a multi-user beamformer (MU-BFer) 2 as well as acentralized backhaul, and a large number (many tens to hundreds) ofRemote Radio Heads (RRHs) 3. Moreover, each RRH is connected to thecentral BBU via a fronthaul connection 4. In an indoor environment, thecost of laying out fronthauls may be similar to the cost of layingbackhauls to densely deployed small cells and may be acceptable in someapplications. However, there are outdoor and some indoor environmentswhere the cost of laying fronthauls is too expensive or impractical. Oneembodiment of this invention uses high speed wireless links to providethe fronthauls to small cells, thus, avoid using extensive wiredconnections such as wired backhauls to densely deployed small cells orfiber fronthauls to a large number of distributed RRHs. The embodimentalso uses a wirelessly broadcast master reference clock signal and eachof the RRHs recovers the master reference clock signal from the wirelessbroadcast and uses it to generate the local clock and carrier signal toensure all RRHs use the same carrier frequency, as well as delay andphase calibrated relative to the master reference clock signal.

The embodiment may further comprising deploying distributed RRHs withfronthauls to form a DM-MIMO network, perform the first-levelbeamforming for each RRH or each cluster of RRHs, then consider allantennas on each of such RRH or cluster of RRHs as a single antenna andperform a second-level beamforming among multiple of such RRHs orclusters of RRHs.

In another embodiment, a MU-MIMO BS with a large number of antennas at acentralized location is employed to provide spatial multiplexedmulti-stream wireless backhaul to distributed RRHs, which form a DM-MIMOnetwork. Beamforming of the DM-MIMO network can be done by a centralMU-BFer by transmitting channel information and baseband samples overthe spatial multiplexed multi-stream wireless backhaul.

In yet another embodiment, a subset of the distributed RRHs are used toperform MU-MIMO beamforming, i.e., a channel matrix that only includesthe channels of the subset of RRHs and selected UEs are used incomputing the precoding or detection matrix by the MU-BFer to beamformto the selected UEs. This is done independent of what the other RRHs aredoing. When two or more subsets of RRHs arc located in areas that have ahigh degree of RF separation, this is, the path loss of RF signals fromRRHs, hence UEs, in one subset to another subset is large, e.g., −20 dB(referred to as RF disjoint subsets), the two or more subsets performMU-MIMO beamforming independently and simultaneously using the samefrequency resources. Furthermore, a group of RF disjoint subsets performMU-MIMO beamforming independently and simultaneously using the samefrequency resources at time slot 1, and another group of RF disjointsubsets perform MU-MIMO beamforming independently and simultaneouslyusing the same frequency resources at time slot 2. The two groups mayoverlap, meaning that some RRHs may belong to more than one group. Thisis spatial and time division of DM-MIMO, alleviating the problem ofpilot contamination in channel estimation and reduces the computationalload for beamforming by reducing dimensions of the channel matrices.

In MU-MIMO beamforming, uncorrelated channels are strongly preferred asthey lead to low condition numbers of the channel matrix and highercapacity. However, in an area with many nearby UEs, e.g., large crowdsin stadiums, live events, etc., some MU-MIMO channels are highlycorrelated. One embodiment is a method of user grouping to improveMU-MIMO beamforming comprising calculating correlations of the channelsof different UEs; selecting UEs with low channel correlation into agroup; allocating frequency resources to the group of UEs; computingprecoding and/or detection matrix using channel matrix of the UEs in thegroup on the allocated frequency resources; and perform MU-MIMObeamforming with UEs in the group on the allocated frequency resources.Furthermore, UEs that are to be served at the same time slot may bedivided into two or more such groups, each of which is allocated a partof the available frequency resources.

In a MU-MIMO system, the delay spreads of channels with different UEsmay differ significantly. A larger delay spread corresponds to a shortcoherence bandwidth, thus requires computation of precoding and/ordetection matrix on a larger number of frequency resource blocks orsubcarriers. If channels with shorter delay spreads and with largerdelay spreads are mixed together in MU-MIMO beamforming, precodingand/or detection matrix must be computed on the finer resolution offrequency resource blocks or subcarriers demanded by the TCs with largerdelay spreads. This wastes computation resources by performingunnecessary computations on smaller frequency resource blocks or groupof subcarriers for channels that have larger coherence bandwidth, and/orreduces the number of UEs that can be simultaneously served on the samefrequency resources due the size of the matrices that can be processedin the allowed time by the hardware. Channels with larger coherencebandwidth only need computation of precoding and/or detection matrix ona small number of frequency resource blocks. One embodiment is a methodof user grouping for efficient MU-MIMO beamforming computationcomprising estimating the delay spreads or coherence bandwidth of thechannels of different UEs; selecting UEs with similar delay spreads orcoherence bandwidth into a group; allocating frequency resources to thegroup of UEs; computing precoding and/or detection matrix using channelmatrix of the UEs in the group on the allocated frequency resources; andperform MU-MIMO beamforming with UEs in the group on the allocatedfrequency resources. Furthermore, UEs that are to be served at the sametime slot may be divided into two or more such groups, each of which isallocated a part of the available frequency resources.

User grouping and frequency allocation also need to consider other UEchannel conditions, i.e., Channel Quality Information (CQI), ChannelEstimation Error (CEE), and UE Speed Indication Information (SII), asdescribed in the Provisional Patent Application entitled “FrequencyResource Allocation in MU-MIMO Systems”, 61/968,647 filed on Mar. 21,2014. One embodiment estimates the channel parameters, including but notlimited to, correlations, delay spreads or coherence bandwidth, CQI, CEEand SII of the channels of different UEs; selecting those UEs into agroup based on their channel parameters such that it improves systemperformance, including but not limited to, improved MU-MIMO beamforming,increased system throughput, reduced computational load, reduced powerconsumption; allocating frequency resources to the group of UEs;computing precoding and/or detection matrix using channel matrix of theUEs in the group on the allocated frequency resources; and performMU-MIMO beamforming with UEs in the group on the allocated frequencyresources. Furthermore, UEs that arc to be served at the same time slotmay be divided into two or more such groups, each of which is allocateda part of the available frequency resources.

Although the foregoing descriptions of the preferred embodiments of thepresent inventions have shown, described, or illustrated the fundamentalnovel features or principles of the inventions, it is understood thatvarious omissions, substitutions, and changes in the form of the detailof the methods, elements or apparatuses as illustrated, as well as theuses thereof, may be made by those skilled in the art without departingfrom the spirit of the present inventions. Hence, the scope of thepresent inventions should not be limited to the foregoing descriptions.Rather, the principles of the inventions may be applied to a wide rangeof methods, systems, and apparatuses, to achieve the advantagesdescribed herein and to achieve other advantages or to satisfy otherobjectives as well.

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 10. (canceled) 11.A method for Distributed Massive MIMO (DM-MIMO) comprising Using one ormore central Baseband Units (BBUs), connected by a backhaul to a corenetwork, to perform baseband signal processing; One or more Multi-UserBeamformers for each Baseband Unit performing multi-user MIMOcomputations for antennas on all Remote Radio Heads (RRHs) connected toit or for antennas on one or more clusters of RRHs connected to it toachieve a high order of spatial multiplexing over the entire coveragearea with low inter-beam interference. A number of RRHs distributed overa geographic area transmitting and receiving wherein each RRH contains aplural of antennas, RF transceivers and a synchronization circuit and isconnected to a Multi-User Beamformer via an optical fiber, cable or awireless link fronthaul connection; and, A BBU listening to the pilotsignals transmitted by one or more UEs covered by a neighboring cell,using the received pilot signals to estimate the channel with the saidUEs, computing a pre-coding matrix to make the transmissions by theBBU's RRHs orthogonal to the channels to the said UEs in the neighboringcell.
 12. A method for Distributed Massive MIMO (DM-MIMO) comprisingUsing one or more central Baseband Units (BBUs), connected by a backhaulto a core network, to perform baseband signal processing; One or moreMulti-User Beamformers for each Baseband Unit performing multi-user MIMOcomputations for antennas on all Remote Radio Heads (RRHs) connected toit or for antennas on one or more clusters of RRHs connected to it toachieve a high order of spatial multiplexing over the entire coveragearea with low inter-beam interference; A number of RRHs distributed overa geographic area transmitting and receiving wherein each RRH contains aplural of antennas, RF transceivers and a synchronization circuit and isconnected to a Multi-User Beamformer via an optical fiber, cable or awireless link fronthaul connection, and, Performing a first-levelbeamforming for each RRH or each cluster of RRHs, then considering allantennas on each of such RRH or cluster of RRHs as a single antenna andperforming a second-level beamforming among multiple of such RRHs orclusters of RRHs.
 13. A method for Distributed Massive MIMO (DM-MIMO)comprising Using one or more central Baseband Units (BBUs), connected bya backhaul to a core network, to perform baseband signal processing; Oneor more Multi-User Beamformers for each Baseband Unit performingmulti-user MIMO computations for antennas on all Remote Radio Heads(RRHs) connected to it or for antennas on one or more clusters of RRHsconnected to it to achieve a high order of spatial multiplexing over theentire coverage area with low inter-beam interference; and, A number ofRRHs distributed over a geographic area transmitting and receivingwherein each RRH contains a plural of antennas, RF transceivers and asynchronization circuit and is connected to a Multi-User Beamformer viaa wireless link fronthaul connection provided by a MU-MIMO BS with alarge number of antennas at a centralized location which transmitschannel information and data wirelessly to the distributed RRHs bybeamforming one or more spatial multiplexed streams to each RRH.
 14. Amethod of user grouping to improve MU-MIMO beamforming comprisingcalculating correlations of the channels of different UEs; selecting UEswith low channel correlation into a group, allocating frequencyresources to the group of UEs; computing precoding and/or detectionmatrix using channel matrix of the UEs in the group on the allocatedfrequency resources; and performing MU-MIMO beamforming with UEs in thegroup on the allocated frequency resource.
 15. The method of claim 14further comprising dividing UEs to be served in the same time slot twoor more such groups, each of which is allocated a part of the availablefrequency resource.
 16. A method of user grouping for efficient MU-MIMObeamforming computation comprising estimating the delay spreads orcoherence bandwidth of the channels of different UEs; selecting UEs withsimilar delay spreads or coherence bandwidth into a group; allocatingfrequency resources to the group of UEs; computing precoding and/ordetection matrix using channel matrix of the UEs in the group on theallocated frequency resources; and performing MU-MIMO beamforming withUEs in the group on the allocated frequency resource.
 17. The method ofclaim 16 further comprising dividing UEs to be served in the same timeslot two or more such groups, each of which is allocated a part of theavailable frequency resource.